I'm trying to design it for ease of assembly. In particular, I'm trying to get it so that you can put things together in just about any order, and so that you don't have to take four things off to extract and replace a fifth.

The first version will have a moving Y platform, like Mendel. But MakerBot have taken Ed Sells' idea of building on a conveyor belt and made it work, so I think replacing the Y motion with a belt will follow soon after.

1200 g•cm is impressive for a NEMA 11. How long is it, and how much does it weigh? If this is for a carriage-mounted extruder and the difference in weight isn't huge, it might make sense to go with a slightly heavier NEMA 14 with a wider mounting point and its center of mass closer to that mounting point.

The NEMA 11 advantage is entirely in its geometry - it is long and thin. It weighs only 5g less than the NEMA 14s I use, but I hope that it will allow a much more compact extruder design. I'll measure it's length when I get home and add that in a comment.

Yes, it does, M2.5. That it adds another type of vitamin, and printing and reaming holes smaller than 3mm would be annoying (why am I such a downer today?)…

This is a geared extruder, right? It probably goes against your "put it together in any order" idea, but it would be cool to use an internal-tooth gear to get a high gear ratio with a small extruder body.

It should be noted that Makerbot's conveyor belts do not run during printing. The conveyor belt sits on top of a moving x/y stage, and it has a DC gearmotor inside to run the belt after the print is finished. I recently got one of these, but have yet to have a successful print while using the conveyor belt attachment. The tension on the belt is quite high, and it would probably take at least a high-torque NEMA 17 to run it at a decent printing speed, though mounting the rollers on real bearings might make a big difference.

I just noticed the diagonal cross-brace on the bottom in the picture. This is a good idea, and something I've been trying to work into a design for a while. That added stiffness might allow you to use a single threaded rod at the front and back instead of two, though this will require a different type of mounting for the Y motor.

Your vertices look like the prettified parametric ones I made for Huxley a while ago, but you might consider using something more like my idea for smaller vertices, found here: http://www.thingiverse.com/thing:5515

Also, some of the newer designs like the emaker huxley are mounting the X-bars vertically, and I'm thinking that you might be able to mount a NEMA 11 between them for a compact carriage/extruder combination.

Also really looking forward to your automated build platform. Personally I'm building a makerbot ABP into my hl-reprap on top of a traditional belt-driven y carriage. However a rr printed solution for replacing the y-carriage all-together would almost certainly be considerably cheaper.

some research later: liking the nema11s but surely servos are the direction we should be headed ultimately? A rc mini servo like this (http://www.hobbyking.com/hobbycity/store/uh_viewItem.asp?idProduct=8742 ) can output 2300g*cm with a much smaller foot-print and has the added benefit that with a openservo board it can stall-detect for homing.